
"""
`module  JAC.PairProduction`  
... a submodel of JAC that contains all methods for computing positron-bound-electron pair production (PEPP) by single-photon 
    absorption cross sections and rates; photon + |i(N)> --> |f(N+1)> + e^+. 
"""
module PairProduction 

using JAC, ..ManyElectron


"""
`struct  PairProduction.Settings`  
    ... defines a type for the details and parameters in computing positron-bound-electron pair production (PEPP) with single-photon 
        absorption (positron) lines; photon + |i(N)> --> |f(N+1)> + e^+.

    + multipoles              ::Array{EmMultipole}           ... Specifies the multipoles of the radiation field that are to be included.
    + gauges                  ::Array{UseGauge}              ... Specifies the gauges to be included into the computations.
    + photonEnergies          ::Array{Float64,1}             ... List of photon energies.
    + printBefore             ::Bool                         ... True, if all energies and lines are printed before their evaluation.
    + selectLines             ::Bool                         ... True, if lines are selected individually for the computations.
    + selectedLines           ::Array{Tuple{Int64,Int64},1}  ... List of lines, given by tupels (inital-level, final-level).
"""
struct Settings 
    multipoles                ::Array{EmMultipole}
    gauges                    ::Array{UseGauge}
    photonEnergies            ::Array{Float64,1} 
    printBefore               ::Bool
    selectLines               ::Bool
    selectedLines             ::Array{Tuple{Int64,Int64},1} 
end 


"""
`PairProduction.Settings()`  ... constructor for the default values of pair-production positron line computations.
"""
function Settings()
    Settings(EmMultipole[], UseGauge[], Float64[], false, false, Tuple{Int64,Int64}[])
end


# `Base.show(io::IO, settings::PairProduction.Settings)`  ... prepares a proper printout of the variable settings::PairProduction.Settings.
function Base.show(io::IO, settings::PairProduction.Settings) 
    println(io, "multipoles:               $(settings.multipoles)  ")
    println(io, "gauges:                   $(settings.gauges)  ")
    println(io, "photonEnergies:           $(settings.photonEnergies)  ")
    println(io, "printBefore:              $(settings.printBefore)  ")
    println(io, "selectLines:              $(settings.selectLines)  ")
    println(io, "selectedLines:            $(settings.selectedLines)  ")
end


"""
`struct  PairProduction.Channel`  ... defines a type for a positron-bound-electron pair production (PEPP) with single-photon absorption
                                        (positron) channel that specifies all quantum numbers, phases and amplitudes.

    + multipole      ::EmMultipole          ... Multipole of the photon absorption.
    + gauge          ::EmGauge              ... Gauge for dealing with the (coupled) radiation field.
    + kappa          ::Int64                ... partial-wave of the free electron
    + symmetry       ::LevelSymmetry        ... total angular momentum and parity of the scattering state
    + phase          ::Float64              ... phase of the partial wave
    + amplitude      ::Complex{Float64}     ... Photoionization amplitude associated with the given channel.
"""
struct  Channel
    multipole        ::EmMultipole
    gauge            ::EmGauge
    kappa            ::Int64
    symmetry         ::LevelSymmetry
    phase            ::Float64
    amplitude        ::Complex{Float64}
end


"""
`struct  PairProduction.Line`  ... defines a type for a pair-production (positron) line that may include the definition of channels.

    + initialLevel   ::Level                  ... initial-(state) level
    + finalLevel     ::Level                  ... final-(state) level
    + positronEnergy ::Float64                ... Energy of the (outgoing free) positron.
    + photonEnergy   ::Float64                ... Energy of the absorbed photon.
    + crossSection   ::EmProperty             ... Cross section for this pair-production (positron) line.
    + channels       ::Array{PairProduction.Channel,1}  ... List of PairProduction.Channels of this line.
"""
struct  Line
    initialLevel     ::Level
    finalLevel       ::Level
    positronEnergy   ::Float64
    photonEnergy     ::Float64
    crossSection     ::EmProperty
    channels         ::Array{PairProduction.Channel,1}
end


"""
`PairProduction.Line()`  ... 'empty' constructor for a pair-production (positron) line between a specified initial and final level.
"""
function Line()
    Line(Level(), Level(), 0., 0., EmProperty(0., 0.), false, PairProduction[] )
end


# `Base.show(io::IO, line::PairProduction.Line)`  ... prepares a proper printout of the variable line::PairProduction.Line.
function Base.show(io::IO, line::PairProduction.Line) 
    println(io, "initialLevel:      $(line.initialLevel)  ")
    println(io, "finalLevel:        $(line.finalLevel)  ")
    println(io, "positronEnergy:    $(line.positronEnergy)  ")
    println(io, "photonEnergy:      $(line.photonEnergy)  ")
    println(io, "crossSection:      $(line.crossSection)  ")
    println(io, "channels:          $(line.channels)  ")
end

end # module
